The present invention relates to strain gage load cells for precision measurement of weights and forces, especially load cells of the Planar Gaged Plate design, and specifically to improvements in the zero adjustments for such load cells.
The most common method for zero adjustment of strain gage bridges uses resistors in series with individual bridge arms. This requires soldering resistors into the strain gage bridge after initial temperature tests. This is a cumbersome process.
It is also known to adjust a strain gage bridge to give zero output at zero load, independent of load cell temperature, by means of two resistors, one shunting a bridge arm, and the other connecting a corner of the strain gage bridge to one terminal of the power supply for the strain gage bridge. These resistors can be connected without opening arms of the strain gage bridge. Consequently, this is a preferred method for zero adjustment.
This preferred method can not be used, however, for zero adjustment of load cells where the sensitivity of individual strain gages must remain constant because the sensitivity equalization will be affected by the zero adjustments. Examples of such load cells and methods for equalizing the sensitivities of individual strain gage elements are disclosed in U.S. Pat. Nos. 3,576,128, 3,968,676 and 4,380,175, and in a copending, commonly-assigned application entitled "Force Measuring Device with Sensitivity Equalization" by Harry E. Lockery.